The present invention relates generally to multichannel record disc recording systems with level control. More particularly, the system superimposes a direct-wave signal and an angle-modulated wave signal and records the same on a record disc. The automatically controlled level of the angle-modulated wave signal increases when the level of the direct-wave signal exceeds a specific level and when it becomes less than another specific level or becomes zero.
The 4-channel record disc recording system is described in U.S. Pat. Nos. 3,686,471 and 3,883,699. Signals from two channels are matrixed, to obtain, respectively, sum and difference signals. Then, a carrier wave is angle modulated responsive to the difference signal. The resulting angle-modulated difference signal is multiplexed with a direct wave sum signal, and the signal thus multiplexed is recorded on the record disc.
If the levels of the respective channel signals are high, interference will readily occur between the high-frequency band component of the direct wave sum signal and the angle-modulated wave difference signal. This interference occurs at the time of recording of the 4-channel record disc and at the time of reproducing by means of a pickup cartridge. In such a case, the interference will give rise to various problems such as disturbance noise, impairment of the S/N ratio, and deterioration of the distortion factor.
Accordingly, one conceivable measure for preventing the occurrence of these adverse effects is to maintain the angle-modulated wave difference signal always at a high level. However, when the level of the angle-modulated wave difference signal is always maintained at a high value, the recording cutter head generates heat. This heat, in turn, requires a special gas cooling means of an elaborate scale and high cost.
Accordingly, a system intended to solve the above described problems has been disclosed in U.S. Pat. No. 3,806,667, entitled FOUR CHANNEL PHONOGRAPH MULTIPLEX RECORDING SYSTEM WITH SIGNAL LEVEL CONTROL. In this system, a device is used for reproducing a magnetic tape on which are recorded program signals which are to be recorded on the disc. An auxiliary head is provided ahead or upstream of the regular reproducing head, as viewed in the tape traveling direction. A signal reproduced by this upstream head is passed through circuits such as an equalizer circuit, a gate circuit, a delay circuit, and an integration circuit, thereby to obtain a control signal which varies in accordance with the level of the reproduced signal. By this control signal, the level of the angle-modulated wave difference signal is automatically controlled.
In this system, however, separate transmission lines are used for the 4-channel signals reproduced through the regular reproducing line and for the control signal obtained by reproduction through the upstream auxiliary head. For this reason, it is necessary to readjust to the level of the control signal when the signal level (cutting level) is set in the 4-channel signal transmission system. Furthermore, in this previous system, the upstream auxiliary head must be provided, and the magnetic tape reproducing device becomes complicated. In addition, there are problems resulting from the complicated nature of the control signal circuit, the large and elaborate scale of the disc cutting apparatus, and the troublesome need for making adjustments.
Accordingly, with the aim of solving these problems, a system has been described in a copending U.S. patent application, Ser. No. 589,557, entitled MULTICHANNEL RECORD DISC RECORDING SYSTEM WITH SIGNAL LEVEL CONTROL. In this system, control causes the level of the angle-modulated wave difference signal to become high when the level of the direct-wave sum signal becomes higher than a specific level. By this provision, interference between the direct-wave sum signal and the angle-modulated wave difference signal is reduced.
However, if the levels of all channel signals are very low, or when there are no signals, there is a decrease in the S/N ratio of the demodulation system for the angle-modulated wave signal.